CN110663146B

CN110663146B - Large power cladding pumping single mode fiber Raman laser

Info

Publication number: CN110663146B
Application number: CN201880032089.6A
Authority: CN
Inventors: 瓦伦丁·盖庞特瑟夫; 伊格尔·山马尔特瑟夫; 尼古拉·普拉特诺夫
Original assignee: IPG Photonics Corp
Current assignee: IPG Photonics Corp
Priority date: 2017-05-15
Filing date: 2018-05-14
Publication date: 2021-09-21
Anticipated expiration: 2038-05-14
Also published as: EP3613113B1; EP3613113A2; CN110663146A; JP2020519963A; WO2019013862A3; JP7177090B2; EP3613113A4; WO2019013862A2; KR102472018B1; US11862926B2; US20210159662A1; KR20200006979A

Abstract

A raman fiber laser source is configured with a feed fiber that delivers MM pump radiation to the inner cladding of a double-clad MM raman fiber laser. The MM pump beam radiation has sufficient power to generate raman scattering in the MM raman fiber to convert the pump radiation into MM signal radiation at a raman shifted wavelength λ ram, which is longer than the wavelength λ pump of the pump radiation. The raman laser source further has a pair of spaced apart reflectors defining between them a resonator for the signal radiation of the first-order stokes wavelength and flanking at least a portion of the MM core of the raman fiber, the MM core being provided with a central core region doped with impurities to enhance the raman process. The reflector and the central core region are dimensioned to correspond to a fundamental mode of MM signal radiation of the Raman fiber output, the MM signal radiation having a power ranging from several kW to several tens of kW and M²The factor is less than or equal to 1.1.

Description

Large power cladding pumping single mode fiber Raman laser

Technical Field

The present disclosure relates to a high power Continuous Wave (CW) raman fiber laser system operable to output a Single Mode (SM) laser beam having a power ranging between several kilowatts (kW) to tens of kW. In particular, the present invention discloses a high power fiber laser pump outputting multimode (MM) pump light, which is end-coupled to a pump capable of outputting M²The cladding of the multimode (MM) Raman fiber of kW-level SM signal light less than or equal to 1.1.

Background

Fiber lasers are used to efficiently convert poor quality pump radiation delivered by multimode Laser Diodes (LDs) into high quality laser beams. In high power fiber lasers, several powerful multimode LDs are typically coupled (e.g., by pump combiners) to the silica cladding of a double-clad active fiber with a core doped with a rare earth element such as (Yb), erbium (Er), etc. Pump radiation guided by the fiber cladding excites the dopants in the core, providing amplification to the core-guided light which is emitted as fundamental transverse mode radiation and has an approximately gaussian beam profile if the geometry of the fiber meets certain conditions. A feature of known all-fiber laser configurations is the production of high quality laser beams over a wide range of output powers.

It is known that lasing in passive fibers is possible due to inelastic raman scattering of pump radiation resulting in amplification of the displaced scattered light. When two laser beams of different wavelengths, pump (pump) light and signal light, propagate together through a raman-active medium, the longer wavelength light (i.e., stokes wave) can undergo optical amplification at the expense of the shorter wavelength pump beam — a phenomenon known as stimulated raman radiation (SRS). One of the unique characteristics of SRS includes: beam clean-up, i.e. brightness enhancement by SRS in MM fiber; and rapid energy transfer between the pump and the raman signal light. SRS thus provides an attractive solution to optically convert MM to SM laser output in both CW and pulsed formats.

Since fiber raman lasers/amplifiers (FRL and FRA) are based on the raman gain induced by pumping in passive fibers, respectively, the lasing characteristics of these devices are fundamentally different compared to rare earth doped fibers, i.e. small quantum defects characterized by first-order stokes, fast response of gain to pumping variations, low background spontaneous emission and lack of photodarkening effects, which is particularly severe in doped active fibers at short wavelengths.

The output power of conventional core-pumped (SM raman) fiber lasers is limited by the availability of high power SM Diode Lasers (DL). The power level is significantly improved by using the MM multi-clad Raman fiber as a gain medium and high-power MM pump light which is end-pumped into the inner cladding of the Raman fiber. One of the many configurations using cladding pumped Raman fibers is reported by Codemard et al in "High power CW clamped pumped Raman fiber laser" Optics Letters/Vol.31, No.15, 8/1/2006. The RFL disclosed herein is characterized by a double-clad raman fiber having an MM core supporting only the Fundamental Mode (FM) of the desired stokes wavelength, a germanium-doped inner cladding, and a silica outer cladding, by laser pumping using an MM fiber. Raman gain occurs throughout the MM core and inner cladding. FM selection is achieved by Fiber Bragg Gratings (FBGs), where the pitch is adjusted for the effective index of FM. As is conventional in the art, the core has an increased refractive index due to the high concentration of dopants, such as germanium, known to enhance the raman process.

The RFL disclosed herein can output several watts in SM. Single mode is achieved by using a true SM output fiber fused to the end of the raman fiber. The cores of the respective raman and output fibers are configured such that the respective Mode Field Diameters (MFDs) of the single and fundamental modes substantially match each other. The raman resonator is defined between strong and weak FBGs and provides gain substantially only to the fundamental mode of the first-order stokes wavelength. However, the SM output is obtained by filtering out the undesirable high-order modes present at the output of the raman fiber using an SM output fiber.

The necessity to dope the entire MM core of the above disclosed raman fibers at the reported high concentration levels adds complexity and cost to the fiber manufacturing process. The reported signal power is far from meeting the current industry demand. However, the applicant is aware of the existence of 1.3kW raman lasers operating in continuous mode (CW). However, to the applicant's knowledge, the laser outputs light M²M with a factor significantly higher than the required quality output²A factor.

Therefore, there remains a need for a MM raman cladding pumped fiber laser source operable to output a beam of mass M²SM bright signal light less than or equal to 1.1 and with power range of several kilowatts.

Disclosure of Invention

The laser source of the present invention meets this need by utilizing a high power fiber laser based pump that outputs MM pump light that is end-coupled into the cladding of the MM raman fiber.

According to one aspect of the present disclosure, a high power Single Mode (SM) raman laser source of the present disclosure is configured with an end-pumped multi-clad raman fiber having an inner cladding that receives multi-mode (MM) pump radiation propagating along a path at a wavelength λ pump and an MM core provided with a central core region. The pump radiation is sufficiently powerful to generate raman scattering to convert the pump radiation into signal radiation at a raman-shifted wavelength or a signal radiation at a signal wavelength λ ram longer than wavelength λ pump.

The disclosed raman laser source also has spaced apart wavelength selective reflectors, such as Fiber Bragg Gratings (FBGs), between which a resonator for the signal radiation is defined. The discriminator and the central core channel are optically aligned and sized to match the FM of the signal radiation output from the Raman fiber, which may reach tens of kilowatts (kW) and M²1.1 or less (preferably 1.05 or less).

The SM raman laser source is further configured with a MM feed fiber located upstream of the raman fiber and delivering pump radiation thereto. The feed fiber and the raman fiber may be directly fused to each other or spaced apart from each other.

In configurations featuring spaced apart feed and MM fibers, the disclosed laser source further includes bulk optics that shape the pump radiation so that it is coupled into the inner cladding of the raman fiber. In addition, tilted mirrors can be placed between the lenses to deflect the back-reflected radiation of the signal wavelength λ ram away from the path to protect the pump lasers.

Drawings

The above and other features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

fig. 1 is a schematic diagram of an all-fiber raman light source of the present invention according to one structural modification.

Fig. 2 is a raman source of the present invention utilizing free space communication between a feed fiber and a raman fiber.

Fig. 3 shows the refractive index profile of the raman fiber of the present invention.

Fig. 4 and 5 show the respective field distribution and intensity distribution of the fundamental mode in the exemplary inventive structures of fig. 1 and 2.

Fig. 6 shows a fiber laser source of the present invention featuring a fiber raman amplifier.

Detailed Description

Reference will now be made in detail to embodiments of the invention. Features of the invention may be used alone or in combination with selected inventive features or all other inventive features in each disclosed configuration of the raman light source of the invention. Wherever possible, the same or similar reference numbers are used in the drawings and the description to refer to the same or like parts or steps. The figures are in simplified form and are not drawn to precise scale. The term "coupled" and similar terms do not necessarily denote a direct or immediate connection, but also include a connection through intermediate elements or devices.

Fig. 1 shows a raman fiber laser source 10 configured with a plurality of SM or MM laser pumps 2, the plurality of SM or MM laser pumps 2 preferably, but not necessarily, being based on fiber lasers each capable of outputting SM pumped bright light with a power up to the kilowatt (kW) level. For example, each SM fiber laser pump 2 is each operable to output 1kW of SM pump light. The SM fiber laser pump may optionally have a Master Oscillator Power Fiber Amplifier (MOPFA) configuration.

The output passive fibres 4 of the respective laser pumps 2 are coupled together in a well-known manner by a fibre combiner having an output fibre 6, which output fibre 6 is also referred to as a feed fibre. The combined beam of multiple pump outputs results in a cumulative pump light of several kilowatts, depending only on the reasonable number of pumps 2 and their respective output powers. For example, a power range of between 5 and 100kW in the CW regime is readily available today. At these powers the beam quality of the accumulated pump light is very good, but it can be improved further as the industrial demand continues to grow. Coupling multiple output fibers to produce a low output or MMThe output combiner is called SM-LM or LM-/MM combiner, which depends on the mode of the individual pump light and the M of the accumulated pump light beam²A factor.

The feed fiber 6 is configured with a cladding 8 surrounding the MM core 12, which directs the accumulated pump light to the MM raman fiber 14, which MM raman fiber 14 is butt-fusion spliced to the output end of the feed fiber 6. The MM Fiber Raman Laser (FRL) is configured to support substantially only the propagation of a single FM by the beam dump effect of SRS in the multimode fiber. Higher pump light power requires the feed fiber 6 to have a larger diameter MM core 12, e.g. 50 to 100 μm, which matches the waveguide inner cladding 16 of the double-clad MM raman fiber 14.

The diameters of the MM raman core 18 and the cladding 16 are chosen to ensure efficient absorption of the pump light over the shortest possible length of the MM raman core. The latter outputs a kW signal light in the fundamental mode at a desired signal light wavelength, preferably the first-order stokes wavelength.

To ensure signal light near the diffraction limit at the desired signal wavelength (e.g., the first-order stokes wave), the raman source 10 includes a central core region 30 in the raman fiber 14 and a combination of the strong and

weak FBGs

26, 28, where the strong and

weak FBGs

26, 28 are each spaced apart for the effective index of the FM. The spaced

FBGs

26 and 28 are written directly into the MM raman core 18 to define a raman cavity for the desired first order stokes wavelength. The central core region 30 is included within the cavity and is doped with an impurity selected from boron, germanium, phosphorus, or a combination thereof. The central region 30 may occupy no more than 70% of the entire core area, which is essentially the core area occupied by FM, while HOMs tend to occupy the periphery of the core area 18. Since the central core region 30 is sized to substantially match the mode field diameter of FM, FM is amplified much more than most HOMs, which are therefore reduced to background noise at the output of Raman fiber 14. Thus, the structure disclosed above ensures that the signal emitted from the raman fiber 14 is radiated in FM.

Optionally, an intermediate MM passive fiber, not shown, may be fused to the opposite ends of the respective feed fiber and raman fiber. Although having an MM core, the intermediate fiber may be configured to support only propagation of FM having an MFD substantially matching the MFD of FM supported in the raman core 18. In this modification, a strong FBG 26 can be written in the intermediate fiber. If pure SM is required, the illustrated structure can have a SM output fiber 24 coupled to the output end of the raman fiber 14. The weak FBG 28 can be written into the SM core 24 of the output fiber 22. The central core region 30 of the raman fiber 14 may remain undoped if both FBGs are formed on the respective intermediate and output passive fibers.

Fig. 2 shows an alternative architecture of the disclosed raman light source 10. As the accumulated pump light from MM feed fiber 6 propagates in free space, it is incident on a guiding optical assembly comprising a collimating lens 25 and a focusing lens 35, respectively spaced apart. As a result, the pump light is focused on the cladding of SM passive fiber 20. A tilted reflector 45 is positioned between

lenses

25 and 35 to prevent back-reflected raman light from propagating upstream of the SM laser pump 2. The rest of the structure is similar to that of fig. 1.

Fig. 3 shows an example of the raman fiber 14 of fig. 1 and 2 having a 30 μm MM core 18 and a central core region 30 of about 20 μm. As can be seen from fig. 4 and 5, the test using the refractive index profile of the raman fiber 14 showed good results, showing the field distribution and intensity distribution of the fundamental mode LPo. The configuration used in the test included a SM fiber laser pump operating at 1070nm, while the SM raman signal light was generated at 1120 nm. All of the above ranges for the optical fibers shown in fig. 1 and 2 are exemplary and can be varied without departing from the scope of the invention. As shown, the refractive index profile of the inner cladding 16 has depressed portions due to the fluorine doped inner region. Alternatively, the cladding 16 and the raised regions of the raman core 18 may be made of pure silica and therefore have substantially the same refractive index.

Fig. 6 illustrates another aspect of the present disclosure, wherein a fiber raman source 50 is configured with a MM Fiber Raman Amplifier (FRA) 32. What is needed to do so is a SM seed laser source 34 configured as a SM fiber laser or pigtailed SM diode laser. The seed 34 outputs a SM signal beam of a desired raman wavelength λ ram, for example 1120nm, which is guided in the SM fiber 36. The wavelength range of the pump wavelength, and thus the signal wavelength, is not limited to a spacing of 1 to 2 microns and extends beyond it.

The seed 34 may include an SM DL or SM fiber laser. The SM fiber 36 can be fused directly to the center fiber of the SM-MM combiner 48 or, as shown, directly to the strong FBG 38, with the strong FBG 38 and the weak FBG 46 together as part of the central SM fiber laser pump 42. Here, as in the schematic diagrams of fig. 1 and 2, the SM-MM combiner is coupled at an input end to the central SM fiber laser pump 42 and the plurality of SM fiber laser pumps 44, respectively, and at an output end to the feed fiber 52. The feed fiber 52 is configured with a core for supporting the SM signal beam and a cladding that guides the cumulative pump beam at a wavelength λ pump shorter than the desired raman wavelength, which may be 1070nm, for example. The end face is fused to double clad FRA 32, the cumulative MM pump beam is coupled into the FRA cladding, and the signal beam is coupled into the FRA MM core.

Alternatively, element 42 of fig. 6 may be configured as a seed laser that outputs light at a first-order stokes wavelength (e.g., 1120 nm). The remaining configuration, except for the assembly 34, remains the same as disclosed above.

As the accumulated pump beam continues to pass through the MM core of FRA 32, its energy is transferred to the first-order stokes wave. The suppression of higher order stokes waves is achieved by the calculated length of the raman fiber and the ratio between MM core and cladding diameters. Amplification of the fundamental mode at the expense of the higher order transverse modes is due to the matching MFDs of the respective feed and raman fibers and their alignment. As in the embodiments of fig. 1 and 2, the MM raman core may be provided with a central region doped with ions of a standard raman dopant, and the central region is dimensioned to correspond to the MFD of the fundamental mode. If the remaining unabsorbed pump light still propagates through the cladding of the raman fiber, a mode stripper may be arranged along the downstream section of the fiber string in any of the embodiments of respective fig. 1, 2 and 6.

Although the present disclosure has been described in terms of disclosed examples, various modifications and/or additions to the above disclosed embodiments will be apparent to those skilled in the laser art without departing from the scope and spirit of the appended claims.

Claims

1. A high power single mode raman laser source comprising:

a multi-clad raman fiber, said raman fiber being end-pumped and having:

an inner cladding that receives pump radiation propagating along a path at a pump wavelength λ pump, the pump radiation being multimode; and

a multimode core surrounded by the inner cladding and provided with a central core region, the pump radiation generating Raman scattering causing conversion of the pump radiation into signal radiation at a Raman-shifted wavelength λ ram, wherein the Raman-shifted wavelength λ ram is larger than the pump wavelength λ pump,

the central core region is sized to confine a fundamental mode of the signal radiation and is doped with an impurity that enhances the Raman scattering; and

spaced apart wavelength selective reflectors defining therebetween a resonator for a fundamental mode of the signal radiation, the resonator at least partially including the central core region,

the wavelength selective reflector is sized to match a fundamental mode of the signal radiation, wherein the Raman fiber outputs the signal radiation in the fundamental mode with a power ranging between a few kilowatts and a few tens of kilowatts and M²≤1.1。

2. The high power single mode raman laser source of claim 1 further comprising a multimode feed fiber located upstream of the raman fiber and delivering the pump radiation to an upstream end of the raman fiber.

3. The high power single mode raman laser source of claim 2, wherein the feed fiber and raman fiber are directly fused to each other, and the wavelength selective reflector is a fiber bragg grating formed in the central core region.

4. The high power single mode raman laser source of claim 2 further comprising a multimode intermediate passive fiber fused to opposite ends of the respective feed and raman fibers and configured with a multimode core supporting propagation of a fundamental mode of the intermediate passive fiber, wherein the fundamental modes of the intermediate passive and raman fibers having respective mode field diameters are matched to each other.

5. The high power single mode raman laser source of claim 4 further comprising a single mode output passive fiber fused to the downstream end of the raman fiber, the wavelength reflectors being respective fiber bragg gratings written into the cores of the respective intermediate and single mode output passive fibers and being optically aligned with the central core region.

6. The high power single mode raman laser source of claim 2 further comprising: a collimating lens and a focusing lens between the spaced apart feed fiber and the raman fiber; and a tilting mirror between the collimating lens and the focusing lens to deflect back reflected light away from the path.

7. The high power single mode raman laser source of claim 1 further comprising a plurality of fiber laser pumps having respective output fibers coupled together in a beam combiner such that the outputs of the respective fiber laser pumps constructively interfere with each other to produce high power multimode pump radiation at a pump wavelength λ pump.

8. The high power single mode raman laser source of claim 1, wherein the pump wavelength λ pump is 1070nm and the raman-shifted wavelength λ ram is 1120 nm.

9. The high power single mode raman laser source of claim 1, wherein the inner cladding and the multimode core of the raman fiber are made of pure silica and have respective indices of refraction matched to each other, the inner cladding region being doped with fluorine to provide the depression.